1. Technical Field of the Invention
The present invention relates to mobile communications systems and, in particular, to determining the geographical position of a mobile station in, for example, a TDMA-type mobile telephony system.
2. Description of Related Art
Several methods for determining the position of a mobile station in a cellular land-based mobile radio system are known in the art.
PCT 9G/00274 describes one such method for determining the position of a mobile station. The position of the mobile station is determined with the aid of triangulation after measuring and recording propagation delays and signal strengths between the mobile station and neighboring base stations. A traffic connection is first established between a mobile station and first base station, and then the signal strength and the propagation delay are measured in respect to this connection. In order to be able to measure the propagation delay and the signal strength between the mobile station and a second base station, it is necessary to handover the radio connection so as to maintain the connection via the second base station. When measuring signal strength and propagation delay in respect to further base stations, it is necessary to effect a new handover.
Upon completion of the measurement run, the radio connection is again switched to the original base station. It will be noted that a complete handover takes place with each base station, i.e., the voice/data traffic, as well as the control information, is transmitted over the connection between the mobile station and the new base station. This means that the connection will be setup over a non-optimal base station. The radio connection is setup over channels which are not intended for use in the cell in which the mobile station is located, a circumstance which is liable to having a disturbing effect on other traffic in the mobile radio system and also to impair the quality of the established connection.
SE-B-466 376 describes another method for determining the position of a mobile station in a Global System for Mobile Communications (GSM)-type mobile telephone system. In this case, it is also possible to determine the position of the mobile station even when the system base stations are not synchronized. A base station measures the time delay upon the arrival of an access signal sent by the mobile station in relation to the expected time arrival of the signal if the mobile station were located in the same place as the base station. The access signal is sent by the mobile station over the random access channel (RACH) common to a radio base station. This channel is used when the mobile station is in an idle mode. The measuring procedure is repeated at several base stations, where three measurement values are required for determining the position of the mobile station.
The mobile station designates the base stations to which an access signal is sent. According to one embodiment, the time delays measured in the base stations are sent back to the mobile station in the form of so-called time-alignment values (TA). The mobile station receives a TA from each designated base station and forwards the TA value together with the identity of the base station to a localizing unit, in which the position of the mobile station is determined. Unfortunately, with this technique, it is necessary to modify the mobile station in order to transmit these data. It is also necessary to modify the base station in order to be able to designate several base stations to which access is initiated. In a further embodiment, the base stations send the measured time delays to the localizing unit.
SE 96005772, a Swedish Patent Application, also describes position determining. It describes how the position of a mobile station in a TDMA-type mobile radio system can be determined by measuring the propagation delay between a mobile station and at least three radio base stations. The mobile station measures the difference in arrival times between signal bursts transmitted from the various base stations. One problem is that the radio base stations are not synchronized time-wise, meaning that their internal signal burst transmission schedules are displaced in time. In order for the propagation delay to be measured, it is necessary to know the time at which a signal is transmitted and the time at which this signal is received by the mobile station. In order to establish the time of transmission, there is used a reference mobile whose position is known; consequently, the propagation delay of signals from neighboring base stations is also known.
The reference mobile also measures the time difference between signal bursts from different base stations. The time differences measured in the reference mobile and in the mobile whose position is to be determined are sent to an evaluating node, which calculates the time displacements between the base stations. Subsequent calculation of the propagation delays between the radio base stations and the mobile, and therefore calculation of the position of the mobile, can be effected either at the evaluation node or at the mobile station.
A non-exhaustive list of objects of the invention follows:
An object of the invention is to enable the position of a mobile station in a mobile radio system to be determined;
Another object of the invention is to enable the position of a mobile station in a mobile radio system to be determined with non-synchronized radio base stations;
Yet another object of the invention is to enable the position of a mobile station in a mobile radio system to be determined without requiring modification to the mobile station;
Still another object of the invention is to enable the position of a mobile station in a mobile radio system to be determined using generally existing functions in the land-based part of the system;
Yet still another object of the invention is to enable the position of a mobile station in a mobile radio system to be determined while limiting the additional radio frequency interference with respect to its own radio traffic and neighboring radio traffic caused by the position determining process; and
Still yet another object of the invention is to enable the position of a mobile station in a mobile radio system to be determined with non-synchronized radio base stations without requiring modification to the mobile station using generally existing functions in the land-based part of the system while limiting the additional radio frequency interference with respect to its own radio traffic and neighboring radio traffic caused by the position determining process.